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Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 8/17/2000 Publication Date: N/A Citation: N/A Interpretive Summary: Calves born to two-year-old heifers experience difficulty at parturition much more often than those born to older cows. Differences in ease of calving appear to be heritable, but several pieces of information are needed before selection can be effective. It is important to know how much the calf and the two-year-old heifer each contribute to ease of calving and how genetic differences are associated with birth weight, weaning weight, and gain after weaning. It is also important to know how scoring systems for ease of calving affect heritability. Scoring systems can range from two scores, assisted vs unassisted at birth, to increasingly finer gradations of the amount of assistance required. Analyses of 5,986 calves born to two-year-old heifers in twelve breeding herds estimated a moderate heritability of the calf's contribution to ease of calving. Calving ease was highly associated with lighter birth weight and less associated with lighter weights at older ages. The heifer's genetic contribution to ease of calving was half that of the calf's. Use of only two scores for ease of calving was predicted to reduce heritability by 23 percent. There was little predicted increase in heritability when more than four scores were used. Results of this research show that selection should be able to reduce calving difficulty. Birth weight was found to be an important correlated trait that should be used for genetic prediction of calving difficulty. Technical Abstract: Heritability of 2-yr-old heifer calving difficulty score was estimated in nine purebred and three composite populations with a total of 5,986 calving difficulty scores from 520 sires and 388 maternal grandsires. Estimates were .43 for direct (calf) genetic effects and .23 for maternal (heifer) genetic effects. The correlation between direct and maternal effects was -.26. Direct effects were strongly positively correlated with birth weight and moderately correlated with 200-d weight and postweaning gain. Smaller negative correlations of maternal calving difficulty with direct effects of birth weight, weaning weight, and postweaning gain were estimated. Calving difficulty was scored as from 1 to 7. Predicted effects of using seven optimal scores or four scores were negligible. The predicted effect of using only two categories was a 23% reduction in heritability. Phenotypic and direct genetic variance increased with increasing average population calving difficulty score. Estimates of direct and maternal heritability for 2-yr-old calving difficulty score are larger than many literature estimates. The current estimates suggest substantial genetic variance for direct and maternal genetic effects. The direct effects of 2-yr-old calving difficulty score appear to be much more closely tied to birth weight than are maternal effects. |
